Vapor permeation is a membrane-based process that can be used to separate mixtures of vapors. In an example of such a process, a vaporous mixture of water containing low concentrations of an organic is fed at essentially ambient pressure to the feed side of a membrane, while a vacuum pump or a gaseous sweep stream maintains a sufficiently low partial pressure of the organic on the permeate side of the membrane to provide a chemical potential gradient of the organic across the membrane. The organic and some of the water are transported to the permeate side of the membrane to form a vapor-phase permeate.
One problem commonly associated with vapor permeation is economically providing and maintaining the chemical potential gradient across the membrane. Those permeation processes employing a vacuum pump or condenser to provide the necessary chemical potential gradient are energy-intensive and thus expensive to operate. As the vapor feed stream passes along the length of the membrane, the concentration (and, therefore, the vapor pressure) of the vaporous component desired to be removed from the vaporous feed stream is reduced to low levels. Thus, to maintain a driving force for transport, the partial pressure of that component in the permeate stream must be kept even lower for permeation and therefore separation to take place. If a vacuum pump is used to maintain the difference in partial pressure of the permeated component in the vapor phase feed stream and the partial pressure of the component in the vapor phase permeate, the pump must maintain a very high vacuum, i.e., a very low absolute pressure, thus incurring high capital and operating costs. Similarly, if a condenser is used, extremely low temperatures must be maintained, requiring a costly and complicated refrigeration system.
Commonly-owned U.S. Pat. No. 5,236,474 discloses a membrane process for the removal of condensable water vapor from air by the countercurrent circulation of a cool extraction fluid such as liquid water at 8.degree. C. on the permeate side of the membrane. The permeate-side cool extraction fluid acts as both a heat extraction fluid and an entrainment fluid that lowers the temperature of the vapor-containing feed gas in direct contact with the feed side of the membrane, thereby condensing vapor from the feed gas. Due to a total pressure differential driving force across the membrane, bulk flow of the liquid condensed vapor through the membrane takes place and the condensed vapor is entrained in the cool extraction fluid.
U.S. Pat. Nos. 4,466,202, 4,553,983, 4,857,078 and 5,089,033 all disclose membrane processes for the removal of organic vapors generally from non-condensable feed streams, but none of the processes uses or suggests the use of a sweep stream.
Thus, although the prior art has utilized a countercurrent cool extraction fluid in a permeation process for the removal of a condensable vapor such as water from a non-condensable feed stream such as air, there has been no recognition of the value of using a condensable countercurrent sweep in a process for the separation of the components of a feed containing a mixture of several vapors, the mixture being predominantly condensable, with both the feed and the permeate being maintained in a non-condensed state. The prior art has, in fact, generally taught away from the use of a condensable countercurrent sweep because the sweep has been thought to dilute the purity of the permeate stream with little or no benefit. Thus, it has not been recognized that the benefits of increased average permeate flux and increased average temperature obtained via the present invention more than offset any decrease in purity of the permeated component.